Concerns for the environment and for energy supplies have resulted in a demand for lighter motor vehicles. It is desirable, therefore, to provide motor vehicle chassis and suspension system components of high strength aluminum alloys. Currently, most automotive chassis and suspension system components are made by assembly of multiples of small parts made by extrusion, hydroforming, welding, etc. The most common materials are cast iron, austenitic ductile iron, or aluminum alloys. The typical minimum yield strength is in the range from 150–190 MPa with a 5 to 10% elongation.
Aluminum casting alloys presently in use contain silicon to improve castability and magnesium to improve the mechanical properties. The presence of magnesium causes the formation of large intermetallic particles which cause reduced toughness. A typical aluminum casting alloy currently in use is A356 with a T6 temper. T6 heat treatment, which has the detrimental effect of causing dimensional changes, is required for such alloys.
The cost of such components is very high due to the many operations involved in their manufacture. These include casting, heat treatment, quench and straightening. To reduce that cost and simultaneously improve product performance, the challenge is to make one piece castings at lower cost that outperform the fabricated products. However, casting processes naturally present problems related to their limitations, which include minimum wall thickness, part distortion from mold ejection, solution heat treatment, and quench. The minimum wall thickness for vehicle component castings is typically 2.5 mm.
Solution heat treatment and quenching are commonly used for castings to achieve adequate mechanical properties. The heat treatment referred to as T6 employs temperatures sufficiently high that brittle eutectic structures are eliminated by solid-state diffusion. Such solution heat treatment introduces distortions due to creep at the high temperatures employed. Quenching introduces distortions due to the residual stresses introduced during the quench. These distortions require correction by machining or by plastic deformation processes. Solution heat treatment and quenching are both expensive. Correction of distortion is also expensive, or may, in large components, be impossible.
The elimination of solution heat treatment and quenching is, therefore, very desirable for vehicle cast products, particularly for large and complex structural components such as subframe, engine cradle, etc. It is, therefore, desirable to provide an alloy which can achieve the required mechanical properties with only a T5 temper, which is a low temperature artificial ageing process. The temperatures used for T5 temper are generally below 200° C. At the low temperatures employed for T5 temper, creep does not cause significant distortion.
It has been found that the need for solution heat treatment is eliminated if constituents which cause large particles are reduced or eliminated from the melt, and elements are added which, during T5 temper, cause fine grain precipitates. The elimination of large particles improves fracture toughness and ductility. The presence of fine grain precipitates provides increased strength.